Advances in dissecting endomembrane trafficking with small molecules

Clathrin-mediated endocytosis facilitates the internalization of Magnaporthe oryzae effectors into rice cells

Fungi and oomycetes deliver effectors into living plant cells to suppress defenses and control plant processes needed for infection. Little is known about the mechanism by which these pathogens translocate effector proteins across the plasma membrane into the plant cytoplasm. The blast fungus Magnaporthe oryzae secretes cytoplasmic effectors into a specialized biotrophic interfacial complex (BIC) before translocation. Here we show that cytoplasmic effectors within BICs are packaged into punctate membranous effector compartments that are occasionally observed in the host cytoplasm. Live cell imaging with fluorescently labeled proteins in rice (Oryza sativa) showed that these effector puncta colocalize with the plant plasma membrane and with CLATHRIN LIGHT CHAIN 1, a component of clathrin-mediated endocytosis (CME). Inhibiting CME using virus-induced gene silencing and chemical treatments resulted in cytoplasmic effectors in swollen BICs lacking effector puncta. By contrast, fluorescent marker co-localization, gene silencing and chemical inhibitor studies failed to support a major role for clathrin-independent endocytosis in effector translocation. Effector localization patterns indicated that cytoplasmic effector translocation occurs underneath appressoria before invasive hyphal growth. Taken together, this study provides evidence that cytoplasmic effector translocation is mediated by clathrin-mediated endocytosis in BICs and suggests a role for M. oryzae effectors in co-opting plant endocytosis.

Identification of a Devernalization Inducer by Chemical Screening Approaches in Arabidopsis thaliana

Vernalization is the promotion of flowering after prolonged exposure to cold. In Arabidopsis thaliana, vernalization induces epigenetic silencing of the floral repressor gene FLOWERING LOCUS C (FLC). The repressive epigenetic mark trimethylation of lysine 27 on histone H3 proteins (H3K27me3) is a critical contributor to the epigenetic silencing of FLC. Interestingly, the deposited H3K27me3 in the FLC locus can be erased by short-term high-temperature treatment. This is referred to as devernalization. In this study, we identified a novel chemical compound, 4-Isoxazolecarboxylic acid, 3,5-dimethyl-2-(4-fluorophenyl)-4-isoxazole carboxylic acid 1-methyl-2-oxoethyl ester named as DEVERNALIZER01 (DVR01), which induces devernalization in Arabidopsis seedlings, by an FLC-luciferase reporter-based high-throughput screening assay. DVR01 decreased the amount of H3K27me3 in the FLC locus in vernalized plants, resulting in the upregulation of FLC in the whole plant, including the vasculature and meristem, where FLC represses floral induction genes. We also showed that a 2-week treatment with DVR01 reverted plants with a vernalized status back to a fully non-vernalized status. Collectively, this study provides a novel structure of DVR01, which modulates devernalization via demethylation of H3K27me3 in the FLC locus.

Progress in using chemical biology as a tool to uncover novel regulators of plant endomembrane trafficking

The regulated dynamic transport of materials among organelles through endomembrane trafficking pathways is essential for plant growth, development, and environmental adaptation, and thus is a major topic of plant biology research. Large-scale chemical library screens have identified small molecules that could potentially inhibit different plant endomembrane trafficking steps. Further characterization of these molecules has provided valuable tools for understanding plant endomembrane trafficking and uncovered novel regulators of trafficking processes.

NHX-type Na+(K+)/H+ antiporters are required for TGN/EE trafficking and endosomal ion homeostasis in Arabidopsis

The regulation of ion and pH homeostasis of endomembrane organelles is critical for functional protein trafficking, sorting and modification in eukaryotic cells. pH homeostasis is maintained through the activity of vacuolar H+-ATPases (V-ATPases) pumping protons (H+) into the endomembrane lumen, and counter-action by cation/proton exchangers such as the NHX family of Na+(K+)/H+ exchangers. In plants, V-ATPase activity at the trans-Golgi network/early endosome (TGN/EE) is important for secretory and endocytic trafficking, however the role of the endosomal antiporters NHX5 and NHX6 in endomembrane trafficking is unclear. Here we show through genetic, pharmacological, and live-cell imaging approaches that double knockout of NHX5 and NHX6 results in the impairment of endosome motility, protein recycling at the TGN/EE, but not in the secretion of integral membrane proteins. Furthermore, we report that nhx5 nhx6 mutants are partially insensitive to osmotic swelling of TGN/EE induced by the monovalent cation ionophore monensin, and to late endosomal swelling by the phosphatidylinositol 3/4-kinase inhibitor wortmannin, demonstrating that NHX5 and NHX6 function to regulate the luminal cation composition of endosomes.

Chemical Activation of EDS1/PAD4 Signaling Leading to Pathogen Resistance in Arabidopsis

In a chemical screen we identified thaxtomin A (TXA), a phytotoxin from plant pathogenic Streptomyces scabies, as a selective and potent activator of FLAVIN-DEPENDENT MONOOXYGENASE1 (FMO1) expression in Arabidopsis (Arabidopsis thaliana). TXA induction of FMO1 was unrelated to the production of reactive oxygen species (ROS), plant cell death or its known inhibition of cellulose synthesis. TXA-stimulated FMO1 expression was strictly dependent on ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1) and PHYTOALEXIN DEFICIENT4 (PAD4) but independent of salicylic acid (SA) synthesis via ISOCHORISMATE SYNTHASE1 (ICS1). TXA induced the expression of several EDS1/PAD4-regulated genes, including EDS1, PAD4, SENESCENCE ASSOCIATED GENE101 (SAG101), ICS1, AGD2-LIKE DEFENSE RESPONSE PROTEIN1 (ALD1) and PATHOGENESIS-RELATED PROTEIN1 (PR1), and accumulation of SA. Notably, enhanced ALD1 expression did not result in accumulation of the product pipecolic acid (PIP), which promotes FMO1 expression during biologically induced systemic acquired resistance. TXA treatment preferentially stimulated expression of PAD4 compared with EDS1, which was mirrored by PAD4 protein accumulation, suggesting that TXA leads to increased PAD4 availability to form EDS1-PAD4 signaling complexes. Also, TXA treatment of Arabidopsis plants led to enhanced disease resistance to bacterial and oomycete infection, which was dependent on EDS1 and PAD4, as well as on FMO1 and ICS1. Collectively, the data identify TXA as a potentially useful chemical tool to conditionally activate and interrogate EDS1- and PAD4-controlled pathways in plant immunity.

High-Throughput Screening of Chemical Compound Libraries for Modulators of Salicylic Acid Signaling by In Situ Monitoring of Glucuronidase-Based Reporter Gene Expression

Salicylic acid (SA) is a vital phytohormone that is intimately involved in coordination of the complex plant defense response to pathogen attack. Many aspects of SA signaling have been unraveled by classical genetic and biochemical methods using the model plant Arabidopsis thaliana, but many details remain unknown, owing to the inherent limitations of these methods. In recent years, chemical genetics has emerged as an alternative scientific strategy to complement classical genetics by virtue of identifying bioactive chemicals or probes that act selectively on their protein targets causing either activation or inhibition. Such selective tools have the potential to create conditional and reversible chemical mutant phenotypes that may be combined with genetic mutants. Here, we describe a facile chemical screening methodology for intact Arabidopsis seedlings harboring the β-glucuronidase (GUS) reporter by directly quantifying GUS activity in situ with 4-methylumbelliferyl-β-D-glucuronide (4-MUG) as substrate. The quantitative nature of this screening assay has an obvious advantage over the also convenient histochemical GUS staining method, as it allows application of statistical procedures and unbiased hit selection based on threshold values as well as distinction between compounds with strong or weak bioactivity. We show pilot screens for chemical activators or inhibitors of salicylic acid-mediated defense signaling using the Arabidopsis line expressing the SA-inducible PR1p::GUS reporter gene. Importantly, the screening methodology provided here can be adopted for any inducible GUS reporter line.

Small molecules unravel complex interplay between auxin biology and endomembrane trafficking

The establishment and maintenance of controlled auxin gradients within plant tissues are essential for a multitude of developmental processes. Auxin gradient formation is co-ordinated via local biosynthesis and transport. Cell to cell auxin transport is facilitated and precisely regulated by complex endomembrane trafficking mechanisms that target auxin carrier proteins to their final destinations. In turn, auxin and cross-talk with other phytohormones regulate the endomembrane trafficking of auxin carriers. Dissecting such rapid and complicated processes is challenging for classical genetic experiments due to trafficking pathway diversity, gene functional redundancy, and lethality in loss-of-function mutants. Many of these difficulties can be bypassed via the use of small molecules to modify or disrupt the function or localization of proteins. Here, we will review examples of the knowledge acquired by the use of such chemical tools in this field, outlining the advantages afforded by chemical biology approaches.

An early secretory pathway mediated by GNOM-LIKE 1 and GNOM is essential for basal polarity establishment in Arabidopsis thaliana

Spatial regulation of the plant hormone indole-3-acetic acid (IAA, or auxin) is essential for plant development. Auxin gradient establishment is mediated by polarly localized auxin transporters, including PIN-FORMED (PIN) proteins. Their localization and abundance at the plasma membrane are tightly regulated by endomembrane machinery, especially the endocytic and recycling pathways mediated by the ADP ribosylation factor guanine nucleotide exchange factor (ARF-GEF) GNOM. We assessed the role of the early secretory pathway in establishing PIN1 polarity in Arabidopsis thaliana by pharmacological and genetic approaches. We identified the compound endosidin 8 (ES8), which selectively interferes with PIN1 basal polarity without altering the polarity of apical proteins. ES8 alters the auxin distribution pattern in the root and induces a strong developmental phenotype, including reduced root length. The ARF-GEF-defective mutants gnom-like 1 (gnl1-1) and gnom (van7) are significantly resistant to ES8. The compound does not affect recycling or vacuolar trafficking of PIN1 but leads to its intracellular accumulation, resulting in loss of PIN1 basal polarity at the plasma membrane. Our data confirm a role for GNOM in endoplasmic reticulum (ER)-Golgi trafficking and reveal that a GNL1/GNOM-mediated early secretory pathway selectively regulates PIN1 basal polarity establishment in a manner essential for normal plant development.

Live cell imaging of FM4-64, a tool for tracing the endocytic pathways in Arabidopsis root cells

Confocal live imaging of the amphiphilic styryl dye FM4-64 is a valuable technique to monitor organelle dynamics and in particular endocytic pathways. After application in plants, FM4-64 immediately stains the plasma membrane and is then integrated on vesicles following endomembrane system-dependent internalization processes. Over time, FM4-64 becomes distributed throughout the full vesicular network from the plasma membrane to the vacuole, including the components of the secretory pathways. Here we provide succinct examples of the many important developmental processes in plants that rely on endocytosis and describe two suitable methods to trace the endocytic pathways in Arabidopsis thaliana root cells based on the uptake of FM4-64.

Techniques for assessing the effects of pharmacological inhibitors on intercellular protein movement

Intercellular protein movement is an important mechanism in plant development. Here we present an integrated protocol that utilizes an inducible system to block plasmodesmata-dependent movement and assessment of fluorescent recovery after photobleaching (FRAP) to identify compounds that influence intercellular protein movement.

Arabidopsis ribosomal proteins control vacuole trafficking and developmental programs through the regulation of lipid metabolism

The vacuole is the most prominent compartment in plant cells and is important for ion and protein storage. In our effort to search for key regulators in the plant vacuole sorting pathway, ribosomal large subunit 4 (rpl4d) was identified as a translational mutant defective in both vacuole trafficking and normal development. Polysome profiling of the rpl4d mutant showed reduction in polysome-bound mRNA compared with wild-type, but no significant change in the general mRNA distribution pattern. Ribsomal profiling data indicated that genes in the lipid metabolism pathways were translationally down-regulated in the rpl4d mutant. Live imaging studies by Nile red staining suggested that both polar and nonpolar lipid accumulation was reduced in meristem tissues of rpl4d mutants. Pharmacological evidence showed that sterol and sphingolipid biosynthetic inhibitors can phenocopy the defects of the rpl4d mutant, including an altered vacuole trafficking pattern. Genetic evidence from lipid biosynthetic mutants indicates that alteration in the metabolism of either sterol or sphingolipid biosynthesis resulted in vacuole trafficking defects, similar to the rpl4d mutant. Tissue-specific complementation with key enzymes from lipid biosynthesis pathways can partially rescue both vacuole trafficking and auxin-related developmental defects in the rpl4d mutant. These results indicate that lipid metabolism modulates auxin-mediated tissue differentiation and endomembrane trafficking pathways downstream of ribosomal protein function.

SNARE VTI13 plays a unique role in endosomal trafficking pathways associated with the vacuole and is essential for cell wall organization and root hair growth in arabidopsis

BACKGROUND AND AIMS

Root hairs are responsible for water and nutrient uptake from the soil and their growth is responsive to biotic and abiotic changes in their environment. Root hair expansion is a polarized process requiring secretory and endosomal pathways that deliver and recycle plasma membrane and cell wall material to the growing root hair tip. In this paper, the role of VTI13 (AT3G29100), a member of the VTI vesicular soluble NSF attachment receptor (SNARE) gene family in Arabidopsis thaliana, in root hair growth is described.

METHODS

Genetic analysis and complementation of the vti13 root hair phenotypes of Arabidopsis thaliana were first used to assess the role of VTI13 in root hair growth. Transgenic lines expressing a green fluorescent protein (GFP)-VTI13 construct were used to characterize the intracellular localization of VTI13 in root hairs using confocal microscopy and immunotransmission electron microscopy.

KEY RESULTS

VTI13 was characterized and genetic analysis used to show that its function is required for root hair growth. Expression of a GFP-VTI13 fusion in the vti13 mutant background was shown to complement the vti13 root hair phenotype. GFP-VTI13 localized to both the vacuole membrane and a mobile endosomal compartment. The function of VTI13 was also required for the localization of SYP41 to the trans-Golgi network. Immunohistochemical analysis indicated that cell wall organization is altered in vti13 root hairs and root epidermal cells.

CONCLUSIONS

These results show that VTI13 plays a unique role in endosomal trafficking pathways associated with the vacuole within root hairs and is essential for the maintenance of cell wall organization and root hair growth in arabidopsis.

Plant vacuole morphology and vacuolar trafficking

Plant vacuoles are essential organelles for plant growth and development, and have multiple functions. Vacuoles are highly dynamic and pleiomorphic, and their size varies depending on the cell type and growth conditions. Vacuoles compartmentalize different cellular components such as proteins, sugars, ions and other secondary metabolites and play critical roles in plants response to different biotic/abiotic signaling pathways. In this review, we will summarize the patterns of changes in vacuole morphology in certain cell types, our understanding of the mechanisms of plant vacuole biogenesis, and the role of SNAREs and Rab GTPases in vacuolar trafficking.

Trafficking modulator TENin1 inhibits endocytosis, causes endomembrane protein accumulation at the pre-vacuolar compartment and impairs gravitropic response in Arabidopsis thaliana

Auxin gradients are established and maintained by polarized distribution of auxin transporters that undergo constitutive endocytic recycling from the PM (plasma membrane) and are essential for the gravitropic response in plants. The present study characterizes an inhibitor of endomembrane protein trafficking, TE1 (trafficking and endocytosis inhibitor 1/TENin1) that reduces gravitropic root bending in Arabidopsis thaliana seedlings. Short-term TE1 treatment causes accumulation of PM proteins, including the BR (brassinosteroid) receptor BRI1 (BR insensitive 1), PIP2a (PM intrinsic protein 2a) and the auxin transporter PIN2 (PIN-FORMED 2) in a PVC (pre-vacuolar related compartment), which is sensitive to BFA (Brefeldin A). This compound inhibits endocytosis from the PM and promotes trafficking to the vacuole, consistent with inhibition of retrieval of proteins to the TGN (trans-Golgi network) from the PVC and the PM. However, trafficking of newly synthesized proteins to the PM is unaffected. The short-term protein trafficking inhibition and long-term effect on plant growth and survival caused by TE1 were fully reversible upon drug washout. Structure-activity relationship studies revealed that only minor modifications were possible without loss of biological activity. Diversity in Arabidopsis ecotypes was also exploited to identify two Arabidopsis accessions that display reduced sensitivity to TE1. This compound and the resistant Arabidopsis accessions may be used as a resource in future studies to better understand endomembrane trafficking in plants.

Auxin biology revealed by small molecules

The plant hormone auxin regulates virtually every aspect of plant growth and development and unraveling its molecular and cellular modes of action is fundamental for plant biology research. Chemical genomics is the use of small molecules to modify protein functions. This approach currently rises as a powerful technology for basic research. Small compounds with auxin-like activities or affecting auxin-mediated biological processes have been widely used in auxin research. They can serve as a tool complementary to genetic and genomic methods, facilitating the identification of an array of components modulating auxin metabolism, transport and signaling. The employment of high-throughput screening technologies combined with informatics-based chemical design and organic chemical synthesis has since yielded many novel small molecules with more instantaneous, precise and specific functionalities. By applying those small molecules, novel molecular targets can be isolated to further understand and dissect auxin-related pathways and networks that otherwise are too complex to be elucidated only by gene-based methods. Here, we will review examples of recently characterized molecules used in auxin research, highlight the strategies of unraveling the mechanisms of these small molecules and discuss future perspectives of small molecule applications in auxin biology.

The use of multidrug approach to uncover new players of the endomembrane system trafficking machinery

Chemical Biology is a strong tool to perform experimental procedures to study the Endomembrane System (ES) in plant biology. In the last few years, several bioactive compounds and their effects upon protein trafficking as well as organelle distribution, identity, and size in plants and yeast have been characterized. Today, several of these chemical tools are widely used to perform mutant screens and establish the trafficking pathway of a given cellular component. This chapter is a guideline to perform multidrug approaches to study the endomembrane system in plant cells. This type of approach is a powerful and useful strategy to thoroughly determine the trafficking of a specific protein as well as to perform mutant screens based on phenotypes produced by drug treatments. On the other hand, a multidrug approach can address the characterization of a new bioactive molecule and find its cellular pathway target. Overall, this approach can unravel mechanisms and identify new players in endomembrane trafficking.

Plant chemical biology: are we meeting the promise?

As an early adopter of plant chemical genetics to the study of endomembrane trafficking, we have observed the growth of small molecule approaches. Within the field, we often describe the strengths of the approach in a broad, generic manner, such as the ability to address redundancy and lethality. But, we are now in a much better position to evaluate the demonstrated value of the approach based on examples. In this perspective, we offer an assessment of chemical genetics in plants and where its applications may be of particular utility from the perspective of the cell biologist. Beyond this, we suggest areas to be addressed to provide broader access and enhance the effectiveness of small molecule approaches in plant biology.

Proteomics of endosomal compartments from plants case study: isolation of trans-Golgi network vesicles

A detailed understanding of endomembrane processes and their biological roles is vital for a complete picture of plant growth and development; however their highly dynamic nature has complicated comprehensive and rigorous studies so far. Recent pioneering efforts have demonstrated that isolation of vesicles in their native state, paired with a quantitative identification of their cargo, offers a viable and practicable approach for the dissection of endomembrane trafficking pathways. The protocol presented in this chapter describes in detail the isolation of the SYP61 trans-Golgi network vesicles from Arabidopsis. With minor alterations, in a few key parameters, it can be adopted to yield a universal procedure for the broad spectrum of plant vesicles.

Protein secretion: how many secretory routes does a plant cell have?

Conventional protein secretion is mediated by the endomembrane system. Secreted proteins are inserted into the endomembrane system through a N-terminal signal peptide and follow the endoplasmic reticulum to the Golgi trafficking pathway en route to the plasma membrane or the extracellular apoplastic space. In mammalian and yeast cells, unconventional secretion has been identified and relatively well studied. Also in plants, evidence of unconventional secretion mechanisms is accumulating. The ever-increasing number of leaderless proteins identified in proteomic studies indicates the importance of unconventional protein secretion in plants. Novel approaches, such as chemical genomics and vesicle proteomics might be able to provide new insights into unconventional protein secretion in plants.

Proteomics of nonmodel plant species

Until recently, large scale proteomic investigations in the plant field have only been possible for a few model species for which the whole genome sequence had been fully determined. In contrast, for many other species with a strong economic interest as sources of human food and animal feed, as well as industrial and pharmacological molecules, little was known about their genome sequence and identifying the proteome in these species was still considered challenging. However, progress has been made as a result of several recent advances in proteomics tools, e.g. in MS technology and data search programs, and the increasing availability of genomic and cDNA sequences from various species. Moreover, next-generation sequencing technologies now make it possible to rapidly determine, at a reasonable cost, the genome or RNA sequence of species not currently considered as models, thus considerably expanding the plant sequence databases. This review will show how these advances make it possible to identify a large set of proteins, even for species for which few sequences are currently available.

Endocytic trafficking towards the vacuole plays a key role in the auxin receptor SCF(TIR)-independent mechanism of lateral root formation in A. thaliana

Plants' developmental plasticity plays a pivotal role in responding to environmental conditions. One of the most plastic plant organs is the root system. Different environmental stimuli such as nutrients and water deficiency may induce lateral root formation to compensate for a low level of water and/or nutrients. It has been shown that the hormone auxin tunes lateral root development and components for its signaling pathway have been identified. Using chemical biology, we discovered an Arabidopsis thaliana lateral root formation mechanism that is independent of the auxin receptor SCF(TIR). The bioactive compound Sortin2 increased lateral root occurrence by acting upstream from the morphological marker of lateral root primordium formation, the mitotic activity. The compound did not display auxin activity. At the cellular level, Sortin2 accelerated endosomal trafficking, resulting in increased trafficking of plasma membrane recycling proteins to the vacuole. Sortin2 affected Late endosome/PVC/MVB trafficking and morphology. Combining Sortin2 with well-known drugs showed that endocytic trafficking of Late E/PVC/MVB towards the vacuole is pivotal for Sortin2-induced SCF(TIR)-independent lateral root initiation. Our results revealed a distinctive role for endosomal trafficking in the promotion of lateral root formation via a process that does not rely on the auxin receptor complex SCF(TIR).

Isolation and proteomic analysis of the SYP61 compartment reveal its role in exocytic trafficking in Arabidopsis

The endomembrane system is a complex and dynamic intracellular trafficking network. It is very challenging to track individual vesicles and their cargos in real time; however, affinity purification allows vesicles to be isolated in their natural state so that their constituent proteins can be identified. Pioneering this approach in plants, we isolated the SYP61 trans-Golgi network compartment and carried out a comprehensive proteomic analysis of its contents with only minimal interference from other organelles. The proteome of SYP61 revealed the association of proteins of unknown function that have previously not been ascribed to this compartment. We identified a complete SYP61 SNARE complex, including regulatory proteins and validated the proteome data by showing that several of these proteins associated with SYP61 in planta. We further identified the SYP121-complex and cellulose synthases, suggesting that SYP61 plays a role in the exocytic trafficking and the transport of cell wall components to the plasma membrane. The presence of proteins of unknown function in the SYP61 proteome including ECHIDNA offers the opportunity to identify novel trafficking components and cargos. The affinity purification of plant vesicles in their natural state provides a basis for further analysis and dissection of complex endomembrane networks. The approach is widely applicable and can afford the study of several vesicle populations in plants, which can be compared with the SYP61 vesicle proteome.

Small molecules present large opportunities in plant biology

Since the introduction of chemical genomics to plant biology as a tool for basic research, the field has advanced significantly. There are now examples of important basic discoveries that demonstrate the power and untapped potential of this approach. Given the combination of protein and small-molecule complexity, new phenotypes can be described through the perturbation of cellular functions that can be linked to growth and developmental phenotypes. There are now clear examples of overcoming functional redundancy in plants to dissect molecular mechanisms or critical pathways such as hormone signaling and dynamic intracellular processes. Owing to ongoing advances, including more sophisticated high-content screening and rapid approaches for target identification, the field is beginning to move forward. However, there are also challenges to improve automation, imaging, and analysis and provide chemical biology resources to the broader plant biology community.

Clusters of bioactive compounds target dynamic endomembrane networks in vivo

Endomembrane trafficking relies on the coordination of a highly complex, dynamic network of intracellular vesicles. Understanding the network will require a dissection of cargo and vesicle dynamics at the cellular level in vivo. This is also a key to establishing a link between vesicular networks and their functional roles in development. We used a high-content intracellular screen to discover small molecules targeting endomembrane trafficking in vivo in a complex eukaryote, Arabidopsis thaliana. Tens of thousands of molecules were prescreened and a selected subset was interrogated against a panel of plasma membrane (PM) and other endomembrane compartment markers to identify molecules that altered vesicle trafficking. The extensive image dataset was transformed by a flexible algorithm into a marker-by-phenotype-by-treatment time matrix and revealed groups of molecules that induced similar subcellular fingerprints (clusters). This matrix provides a platform for a systems view of trafficking. Molecules from distinct clusters presented avenues and enabled an entry point to dissect recycling at the PM, vacuolar sorting, and cell-plate maturation. Bioactivity in human cells indicated the value of the approach to identifying small molecules that are active in diverse organisms for biology and drug discovery.

Sortin1-hypersensitive mutants link vacuolar-trafficking defects and flavonoid metabolism in Arabidopsis vegetative tissues

Sortin1 is a chemical genetic-hit molecule that causes specific mislocalization of plant and yeast-soluble and membrane vacuolar markers. To better understand its mode of action, we designed a Sortin1-hypersensitive screen and identified several Sortin1-hypersensitive and flavonoid-defective mutants. Mechanistically, Sortin1 mimics the effect of the glutathione inhibitor buthionine sulfoximine and alters the vacuolar accumulation of flavonoids, likely blocking their transport through vacuole-localized ABC transporters. Structure-activity relationship studies conducted in Arabidopsis revealed the structural requirements for Sortin1 bioactivity and demonstrated that overlapping Sortin1 substructures can be used to discriminate between vacuolar-flavonoid accumulations and vacuolar-biogenesis defects. We conclude that Sortin1 is a valuable probe for dissecting novel links among flavonoid transport, vacuolar integrity, and the trafficking of vacuolar targeted cargoes in Arabidopsis.

Protein-Protein Interaction Network and Subcellular Localization of the Arabidopsis Thaliana ESCRT Machinery

The endosomal sorting complex required for transport (ESCRT) consists of several multi-protein subcomplexes which assemble sequentially at the endosomal surface and function in multivesicular body (MVB) biogenesis. While ESCRT has been relatively well characterized in yeasts and mammals, comparably little is known about ESCRT in plants. Here we explored the yeast two-hybrid protein interaction network and subcellular localization of the Arabidopsis thaliana ESCRT machinery. We show that the Arabidopsis ESCRT interactome possesses a number of protein-protein interactions that are either conserved in yeasts and mammals or distinct to plants. We show also that most of the Arabidopsis ESCRT proteins examined at least partially localize to MVBs in plant cells when ectopically expressed on their own or co-expressed with other interacting ESCRT proteins, and some also induce abnormal MVB phenotypes, consistent with their proposed functional role(s) as part of the ESCRT machinery in Arabidopsis. Overall, our results help define the plant ESCRT machinery by highlighting both conserved and unique features when compared to ESCRT in other evolutionarily diverse organisms, providing a foundation for further exploration of ESCRT in plants.